Field of the Invention
The invention is in the field of reproduction technology and is directed to an apparatus for splitting a light beam into two sub-beams.
Such an apparatus for beam splitting can be employed, for example, in a light beam scanner unit for originals scanner devices or recording devices.
In an originals scanner device, also referred to as an input scanner, the light beam sweeps the original to be scanned point-by-point and line-by-line, and the scan light reflected from or allowed to pass by the original is converted into an image signal in an optoelectronic transducer.
In a recording device, also referred to as a recorder, exposer or output scanner, the light beam is intensity-modulated by an image signal, and the intensity-modulated light beam is conducted over a light-sensitive recording material point-by-point and line-by-line.
In a scanner or recorder device of the flatbed type, the holder for the original or, the recording material is a flat surface moved relative to the light beam scanner unit that the light beam sweeps point-by-point and line-by-line.
In a scanner or recorder device of the inside drum type, the holder for the original or, the recording material is designed as a stationary trough shaped like a cylinder segment. The light beam scanner unit moves parallel to the longitudinal axis of the trough, and the light. beam is radially conducted over the trough point-by-point and line-by-line perpendicular to the longitudinal axis.
DE-A-41 28 468 already discloses a light beam scanner unit with an apparatus for splitting a light beam into two sub-beams in a recording device of the inside drum type. The beam splitter apparatus comprises essentially a stationary light source and a deflection unit that turns around an optical axis. The deflection unit is composed of a polarization beam splitter, a polarization converter and a plane mirror.
The polarized light coming from the stationary light source is divided into two sub-beams in the rotating deflection unit by the polarization beam splitter, these two sub-beams being intensity-modulated by an image signal. The two intensity-modulated sub-beams emerge radially offset from the deflection unit, are focussed onto a recording material fixed in an exposure trough and expose the recording material point-by-point and line-by-line. Two lines on the recording material are exposed with the two intensity-modulated sub-beams per revolution, a high recording speed being achieved as a result thereof.
In order to also achieve a high recording quality, the two sub-beams dare not exhibit any positional offset in the recording plane perpendicular to the line direction compared to a rated position in which the two sub-beams are congruent. As a result of such a positional offset, namely, the two lines exposed on the recording material would not proceed equidistantly from one another, and disturbing fluctuations in tonal value that considerably diminish the recording quality would result.
The known beam splitter apparatus has the disadvantage that the light beam generated in the stationary light source must be very precisely adjusted in the direction of the optical axis or, of the rotational axis of the deflection unit so that, after half a revolution of the deflection unit, the sub-beams are incident on a line in the recording plane perpendicular to the line direction without positional offset. When the light beam in the known beam splitter apparatus is not aligned exactly, the two sub-beams, however, exhibit an oppositely directed positional offset relative to the rated position, i.e. the beam positions lies to the left and right of the rated position, as a result whereof a relatively big positional error arises. An exact adjustment, however, is involved and is lost over time, so that a re-adjustment is potentially required. Disturbing positional offsets of the sub-beams also arise when the polarization beam splitter and the plane mirror in the deflection unit are not aligned exactly relative to one another due to manufacturing tolerances. For compensation of errors that have arisen due to an imprecise alignment of the optical components, the known beam splitter apparatus comprises, for example, an adjustable optical adjustment mechanism in the form of a camera wedge arranged in the beam path between light source and deflection unit. Such an adjustment, however, is relatively involved and imprecise, since it can only be carried out when the beam splitter apparatus is not rotating.